Sin begins in the heart.
"Raca" means "emptyhead". If one man called another by this name, he could be sued for libel by the council (also called the Sanhedrin). This would be his judgment.
Jesus was saying that the judgment facing one who called another a fool, would come from God. This didn't mean that someone who called another by this name would suffer eternity in hell, but he would worsen his condition before God on Judgment Day in which works would be measured by fire.
A look at 1 Corinthians 3:12-15 gives a more detailed explanation of this,
Now if anyone builds on this foundation with gold, silver, precious stones, wood, hay, straw, each one's work will be clear; for the Day will declare it., because it will be revealed by fire; and the fire will test each one's work, of what sort it is. If anyone's work which he has built on it endures, he will receive a reward. If anyone's work is burned, he will suffer loss; but he himself will be saved, yet so through fire.
So we see here that the fire referred to is the fire that tests our works and how we treat and speak of, to, and about others clearly falls under "works". Christians should never call one another names or bad-mouth one another. We are to encourage and uplift one another and direct one another back onto the right path if we see our brother or sister leaving that path.
23"Therefore if you bring your gift to the altar, and there remember that your brother has something against you, 24leave your gift there before the altar, and go your way. First be reconciled to your brother, and then come and offer your gift."
We are to love one another and always be quick to forgive. The greatest commandment is love. If we regulary "kill the flesh" as we are called to do; if were are truly capable of loving others as Christ called us to, then we should not find forgiveness to be a hard task. We should desire to make right and correct wrongs with one another. We are all guilty of hurting others and we all need to be ready to forgive.
Bitterness in the heart does not show a heart ready to give to God. God will not receive from us when we are in a state of bitterness toward another.
25"Agree with your advesary quickly, while you are on the way with him, lest your advesary deliver you to the judge, the judge hand you over to the officer, and you be thrown into prison. 26Assuredly, I say to you, you will be no means get out of there till you have paid the last penny."
In the physical aspect of this, Christians should not sue in the courts. Christians are to be like-minded in the things of Christ and be able to handle their matters through love, prayer and compromise, outside of the courts.
In the spiritual tone of this passage, the judge most likely represents God and we should be reconciled to the Father before our judgment comes.
